1. FIELD OF INVENTION:
The present invention relates to radioactive well logging.
2. DESCRIPTION OF PRIOR ART:
It has been known in radioactive well logging to measure and log thermal neutron decay or thermal neutron lifetime in the formation surrounding the borehole, as in U.S. Pat. No. 3,733,486. However, for highly accurate quantitative interpretation of this type of log, it is often necessary to know accurately certain parameters, such as formation porosity, fluid salinity, and shale fraction of the formation as well as the macroscopic thermal neutron capture cross section, .SIGMA., of the formation matrix. Where certain of these parameters could only be roughly estimated, quantitative interpretation of this type of log is often unsatisfactory.
In other types of logging, as in U.S. Pat. No. 3,772,513 to distinguish oil zones from water zones in the formations, also known as chlorine logging, only formation water salinity is required to be accurately known, since a satisfactory interpretation of the log can generally be made with only an estimate of formation porosity. However, this type of log is subject to errors caused by variations of borehole effects or parameters, such as those resulting from washouts, poor cementation, borehole size variations and the like. In contrast, the thermal neutron decay log is less sensitive to borehole effects and generally has greater depths of investigation horizontally into the formation than the chlorine log.
Thus, the two types of logs discussed above, although they can essentially serve as substitutes for each other, are also complimentary in several ways. However, the expense of running both logs on the same well, requiring a separate run of two different logging tools, has, in the past, been too high to justify the more definite results which would be produced.